This invention relates to an optical cable for a customer-use to be connected between an outdoor optical cable line and private residence or the like, and especially to an optical cable having good reliability through the elimination of the effects of stress on an optical fiber core of the cable arising as a result of a curving tendency of a tension member of the cable, and to a method for manufacturing thereof.
There have been optical cables for a customer-use consisting of a tension member such as a steel wire and an optical fiber core disposed in parallel and covered with a sheath made of plastic or the like, for example, having the structure shown in FIG. 4A (Japanese Unexamined Patent Publications Nos. H.10-010384, H.3-156410).
However, in this kind of optical cables for a customer-use, when the tension member has a curving tendency, if the sheath softens due to being heated to a high temperature by the condition of the environment in which the optical cable was installed, for example, by being exposed under released heat from electrical equipment, the curving tendency of the tension member prevails, and consequently stress acts upon the optical fiber core, and damage such as increased transmission loss or breakage of the optical fiber core occurs.
This phenomenon will now be described specifically, using the drawings.
When the sheath 4 of the optical cable shown in FIG. 4A softens by heating, the curving tendency of the tension member 1 may prevail and deformation shown in FIGS. 4B and 4C may occur.
When the optical fiber core 2 is pushed to the inner side of the arc of a tension member 1 whose curving tendency has prevailed, as shown in FIG. 4B, the optical fiber core 2 suffers stress caused by irregular contracted deformation, and its light transmission loss increases. When the optical fiber core 2 is pushed to the outer side of the arc of a tension member 1 whose curving tendency has prevailed, tension acts on the optical fiber core 2, causing possible breakage of the optical fiber core.
Typical types of curving tendency of the tension member 1 of an optical cable are shown in FIGS. 6A, 6B and 6C.
Here, the direction of the curving tendency of the tension member 1 is shown by the direction xe2x86x92R from a tension member 1a, whose curving tendency has prevailed in a natural state wherein it is free from constraint by external force, to its center of curvature O, and the degree of the curving tendency is shown by the size of the radius of curvature R. The direction xe2x86x92R of a curving tendency according to this definition is naturally included in the flat plane (hereinafter called the base plain) containing the tension member 1 and the center of curvature O.
The base plain is shown in FIGS. 6A, 6B and 6C with the reference numerals 200, 300 and 400, respectively.
If the curving tendency is the same along the entire length of the tension member 1, the directions xe2x86x92R of the curving tendencies of all parts of the tension member 1 point toward the same center of curvature O, and their sizes R also take the same value.
When the tension member is wound on a bobbin or straightened under tension or supported by a sheath, this curving tendency is largely constrained by external force and is latent, but when it is freed from external force for example due to the sheath of an optical cable being heated and softening after the cable is installed, the curving tendency prevails and the aforementioned damage occurs.
Hereinafter, except when otherwise indicated, the tension member 1 is referred to in a state such that its curving tendency is constrained by some external force and is completely latent.
The type of curving tendency shown in FIG. 6A is such that the optical fiber core 2 is disposed in a flat plane 220 adjacent to and parallel with the base plane 200 containing the tension member 1 and the direction xe2x86x92R and center of curvature O of its curving tendency, and in the position of the tension member 1 as projected perpendicularly onto that plane. The optical cable shown in FIG. 1 is in the category of the curving tendency type of this FIG. 6A.
In FIG. 6A, the reference numerals 1, 11 denote the tension member with its curving tendency constrained; 2, 12b the optical fiber core in a state corresponding to that; 1a the tension member 1, 11 with its curving tendency having prevailed; and 2a the optical fiber core 2, 12b in a state corresponding to that.
The type of curving tendency shown in FIG. 6B is such that the optical fiber core 2 is in a base plane 300 containing the tension member 1 and the direction xe2x86x92R and center of curvature O of its curving tendency, and the optical fiber core 2 is disposed in parallel with the tension member 1 and on the same side thereof as the direction xe2x86x92Ra of its curving tendency. The deformation of an optical cable due to its curving tendency prevailing as shown in FIG. 4B is of the category of the curving tendency type shown in this FIG. 6B.
The type of curving tendency shown in FIG. 6C is such that the optical fiber core 2 is in a base plane 400 containing the tension member 1 and the direction xe2x86x92R and center of curvature O of its curving tendency, and the optical fiber core 2 is disposed in parallel with the tension member 1 but on the opposite side thereof from the direction xe2x86x92Rb of its curving tendency. The deformation of an optical cable deforming due to its curving tendency prevailing as shown in FIG. 4C is of the category of the curving tendency type shown in this FIG. 6C.
Optical cables have been manufactured by the kind of method shown in FIGS. 7A and 7B. For example, a tension member 1 is given a downward curving tendency by a tension member supply 20, and is supplied together with an optical fiber 12b disposed below it to an extrusion sheathing apparatus. In the extrusion sheathing apparatus, the tension member 1 and the optical fiber 12b are integrated by a sheath being formed on them by a crosshead 40 having a die 40a with a skittle-shaped opening 40b such as that shown in FIG. 7C, after which they are taken up on a takeup drum 60 as a completed optical cable.
An optical cable manufactured by this process is of the category of the curving tendency type shown in FIG. 6B. If either the tension member supply 20 or the die 40a shown in FIGS. 7A and 7B are vertically inverted, the positional relationship between the curving tendency of the tension member 1 and the optical fiber core 2 is inverted and an optical cable of the category of the curving tendency type shown in FIG. 6C is manufactured. Thus, optical cables have normally been of the category of the curving tendency type shown in FIG. 6B or FIG. 6C.
The types of curving tendencies of optical cable shown in FIGS. 6A through 6C are merely typical, and an infinite number of types intermediate between these can also exist, depending on the positional relationship between the optical fiber core 2 and the direction of the curving tendency of the tension member 1. And different curving tendencies can exist in the same optical cable at different positions in the length direction of the optical cable.
When an optical cable has the type of curving tendency of the tension member 1 shown in FIG. 6B or 6C, there is the adverse effects on the optical fiber core 2 as shown in FIGS. 4B and 4C due to prevailing this tendency. On the other hand, when an optical cable has the type of curving tendency of the tension member 1 shown in FIG. 6A, there is almost no adverse effect on the optical fiber core 2.
Although it has been possible for an optical cable of related art coincidentally to include the type of curving tendency shown in FIG. 6A, the technological ideal of deliberately, actively providing the tension member 1 with this type of curving tendency over the entire length of the optical cable to protect the optical fiber core has not been proposed before.
(Hereinafter, the vertical direction refers to the direction in which gravity acts, and the horizontal direction refers to a direction perpendicular to the vertical direction.)
An object of the present invention is to provide a highly reliable optical cable by preventing adverse effects produced on an optical fiber core when a curving tendency of a tension member prevails, by manufacturing the optical cable in such a way that the optical cable is of the category of the curving tendency type shown in FIG. 6A over its entire length.
To achieve this object and other objects, the invention provides an optical cable comprising at least one optical fiber core and a tension member and a sheath, wherein the tension member has a curving tendency and when the flat plane containing the curve formed by the tension member due to its curving tendency is taken as a base plane, the optical fiber core is disposed in the proximity of the position of the image formed when the tension member is projected perpendicularly with respect to the base plane onto a flat plane separate from and parallel with the base plane.
The invention also provides a method for manufacturing an optical cable by disposing in parallel and covering with a sheath by an extruder at least one optical fiber core and a tension member having a curving tendency, wherein when the flat plane containing the curve formed by the tension member due to its curving tendency is taken as a base plane, the sheath is formed with the optical fiber core disposed in the proximity of the position of the image formed when the tension member is projected perpendicularly with respect to the base plane onto a flat plane separate from and parallel with the base plane.
That is, an optical cable according to the invention is made by integrating a tension member 1 and an optical fiber core 2 by forming on them a sheath 4 with the tension member 1 extended in a horizontal direction after having been positively provided with a curving tendency of a fixed radius of curvature R in a horizontal direction and with the optical fiber core 2 disposed in a flat plane 220 adjacent to and parallel with the base plane 300 containing the tension member 1 and the center of curvature O of its curving tendency and in the position of the tension member 1 as projected perpendicularly onto that plane, as shown in FIG. 1A and FIG. 6A.
In the case of an optical cable containing an optical fiber ribbon, in addition to these conditions, effects of the curving tendency prevailing are avoided by the alignment line (500 in FIG. 2C), i.e. the line obtained by joining the centers of the optical fibers at the both sides of the ribbon, being disposed perpendicular to the base plane.
As a result of this construction, due to the relation with the direction xe2x86x92R of the curving tendency of the tension member 1, an optical cable according to this invention curves in the manner shown in FIG. 1B over the entire length of the optical cable, even when the curving tendency of the tension member 1 prevails after the optical cable is manufactured.
In this case, although tensile and compressive stresses act respectively upon the sheath on the outer side and the inner side of the arc of the curved optical cable, in the central position where the optical fiber core is located, these stresses cancel out and have no substantial effect upon the optical fiber core.
As described above, even when the curving tendency of the tension member prevails and the optical cable curves, all that happens is that the tension member and the optical fiber core curve with the same curvature, and the optical fiber core does not suffer stress due to either compression or tension.
Thus, a highly reliable optical cable which suffers neither increased transmission loss nor breakage can be realized.
FIG. 1A is a sectional view showing the construction of an optical cable according to the invention, together with the direction xe2x86x92R of a curving tendency of a tension member thereof, and FIG. 1B is a perspective view showing the optical cable having heat-deformation due to prevailing of the curving tendency of the tension member;
FIG. 2A is a sectional view showing the construction of an optical cable according to the invention containing optical fiber ribbons, together with the direction xe2x86x92R of a curving tendency of a tension member thereof, and FIG. 2B is a perspective view showing the optical cable having heat-deformation due to prevailing the curving tendency of the tension member, and FIG. 2C is a sectional view illustrating the alignment line of an optical fiber ribbon having four optical fibers;
FIG. 3A is a side view and FIG. 3B is a plane view of an apparatus for manufacturing an optical cable according to the invention, and FIG. 3C is a sectional view showing the shape of an opening of a die of an extruder;
FIG. 4A is a sectional view showing the construction of an optical cable of related art together with directions xe2x86x92Ra and xe2x86x92Rb of a curving tendency of a tension member thereof, FIG. 4B is a perspective view showing the optical cable having heat-deformation in the direction xe2x86x92Ra, i.e. to the side of the tension member on which an optical fiber core of the optical cable is disposed, and FIG. 4C is a perspective view showing the optical cable having heat-deformation in the direction xe2x86x92Rb, i.e. to the opposite side of the tension member from that on which the optical fiber core of the optical cable is disposed;
FIG. 5A is a sectional view of an optical cable serving as a second comparison example together with the direction xe2x86x92Ra of a curving tendency of a tension member thereof and FIG. 5B is a perspective view showing the optical cable having heat-deformation;
FIGS. 6A through 6C are perspective views illustrating different types of curving tendency of a tension member in an optical cable. FIG. 6A is a case of an optical cable with a tension member having a horizontal curving tendency of direction xe2x86x92R and an optical fiber core disposed in a flat plane 220 adjacent to and parallel with a base plane 200 containing the tension member and the center of curvature O of its curving tendency and in the proximity of the position of the tension member as projected perpendicularly onto that plane. FIG. 6B is a case of an optical cable with a tension member having a downward curving tendency of direction xe2x86x92Ra and an optical fiber core disposed parallel with the tension member on the side thereof of the direction of the curving tendency. FIG. 6C is a case of an optical cable with a tension member having a downward curving tendency of direction xe2x86x92Rb and an optical fiber core disposed parallel with the tension member on the opposite side thereof from the direction of the curving tendency;
FIG. 7A is a side view, and FIG. 7B is a plane view of an apparatus for manufacturing an optical cable of related art, and FIG. 7C is a sectional view showing the shape of an opening of a die of an extruder.